This invention relates to a method of reducing the content of the meta isomer of a halo substituted alkylbenzene in a mixture with other isomers. In particulars, it relates to exposing the mixture to a brominating agent under conditions such that the meta isomer is preferentially brominated.
Commercial parachloroethylbenzene (PCEB) is made by chlorinating ethylbenzene (EB). After distilling off about 20 wt % of the unreacted EB, the remaining material contains about 50 wt % orthochloroethylbenzene (OCEB), 1.5 wt % metachloroethylbenzene (MCEB), 45 wt % PCEB, and the remainder overchlorinated impurities (about 2.5 wt %). PCEB is used as an intermediate in the preparation of pharmaceuticals, paint pigments, herbicides, and other chemicals. While the presence of small amounts of the OCEB is usually innocuous, it has been found that the presence of MCEB can deleteriously affect the properties of the chemicals made from PCEB. Unfortunately, the boiling point of MCEB is close to the boiling point of PCEB and the two isomers cannot be easily separated.
In U.S. Pat. No. 4,827,058, herein incorporated by reference, a chlorotoluene isomeric mixture is chlorinated in the presence of a Friedel-Crafts catalyst at a temperature of 0.degree. C. up to the boiling point of the mixture. The metachlorotoluene (MCT) chlorinates to dichlorotoluene to a much greater extent than does the orthochlorotoluene (OCT) or the parachlorotoluene (PCT). The PCT-OCT mixture is then separated from the higher boiling dichlorotoluenes (DCT) by distillation.